There is no difference between the two different templates other than the code they put into the file by default. Like you noticed, they both have the same extension and they are both plain text files – the compiler does not treat them any differently.
The general rule of thumb is to only import a module if you need it in that file – basically you get a compile error otherwise. You will get a compiler error because of missing imports if you try to use types from them.
Foundation is basically the Apple development standard library, implemented in Objective-C. It includes data types like NSArray, NSString, and NSDictionary. However, Swift provides replacements for these data types: Array, String, and Dictionary. You should only need to
import Foundation in files that try to interact with Objective-C APIs. However, since all existing APIs are written in Objective-C (since Swift is so new), you will have to import Foundation in almost all files except for basic domain objects.
UIKit actually includes Foundation. If you import UIKit, you do not need to also import Foundation. UIKit is a collection of standard APIs specific to the iOS interface. It includes views like UITableView and controllers like UIViewController. You will need to import this whenever working on the UI of iOS apps.
Cocoa Touch Class
This template is useful for subclassing classes from UIKit and potentially having Xcode generate interface builder files for you. If you don’t need either of those things, you should just use the Swift Class template. Only have your object inherit from NSObject if you really have to (if you need to be able to use your Swift class from Objective-C). If you don’t inherit from NSObject, there are extra optimizations that the compiler can make and it avoids some of the slower parts of the Objective-C runtime.